1. Field of the Invention
The present invention relates generally to ink-jet printing and, more specifically to a method and apparatus for automated optical determination of optimized energy requirements for firing ink droplets from an ink-jet printhead, producing high quality printing while preserving printhead life.
2. Description of Related Art
The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No.1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy [sic] Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
FIG. 1 depicts an ink-jet hard copy apparatus, in this exemplary embodiment, a computer peripheral, color printer, 101. A housing 103 encloses the electrical and mechanical operating mechanisms of the printer 101. Operation is administrated by an electronic controller (usually a microprocessor or application specific integrated circuit ("ASIC") controlled printed circuit board, not shown, but see FIGS. 1A and 3) connected by appropriate cabling to a computer (not shown). It is well known to program and execute imaging, printing, print media handling, control functions, and logic with firmware or software instructions for conventional or general purpose microprocessors or ASIC's. Cut-sheet print media 105, loaded by the end-user onto an input tray 107, is fed by a suitable paper-path transport mechanism (not shown) to an internal printing station where graphical images or alphanumeric text are created using state of the art color imaging and text rendering techniques. A carriage 109, mounted on a slider 111, scans the print medium. An encoder strip and its appurtenant devices 113 are provided for keeping track of the position of the carriage 109 at any given time. A set 115 of individual ink-jet pens, or print cartridges 117A-117D are releasably mounted in the carriage 109 for easy access and replacement; generally, in a full color system, inks for the subtractive primary colors, cyan, yellow, magenta (CYM) and true black (K) are provided. Each pen or cartridge has one or more printhead mechanisms (not seen in this perspective) for "jetting" minute droplets of ink to form dots on adjacently positioned print media. Once a printed page is completed, the print medium is ejected onto an output tray 119.
In essence, the ink-jet printing process involves dot-matrix manipulation of droplets of ink ejected from a pen onto an adjacent print medium (for convenience of explanation, the word "paper" is used hereinafter as generic for all forms of print media). An ink-jet pen 117.sub.x includes a printhead which consists of a number of columns of ink nozzles. Each column (typically less than one-inch in total height) of nozzles selectively fires ink droplets (typically only several picoliters in liquid volume) from addressed nozzles that are directed to create a predetermined print matrix of dots on the adjacently positioned paper as the pen is scanned across the media. A given nozzle of the printhead is used to address a given vertical print column position, referred to as a picture element, or "pixel," on the paper. Horizontal positions on the paper are addressed by repeatedly firing a given nozzle as the pen is scanned across its width. Thus, a single sweep scan of the pen can print a swath of dots. The paper is stepped to permit a series of contiguous swaths. Dot matrix manipulation is used to form alphanumeric characters, graphical images, and even photographic reproductions from the ink drops. Generally, the pen scanning axis is referred to as the x-axis, the paper transport axis is referred to as the y-axis, and the ink drop firing direction is referred to as the z-axis.
Within a thermal ink-jet printhead--in the state of the art having such small dimensions that thin film, integrated circuit fabrication techniques are employed in manufacture--a set of ink drop generators includes individually activated ink heater resistors subjacent the ink firing nozzles. An attribute of printing is the minimum energy required for a given printhead to eject an ink drop, also known as turn-on energy, "TOE." Due to design manufacturing tolerance variations, TOE can vary significantly for a particular pen design specification. Therefore, a printer must provide ink drop firing pulses to fire a compatible pen having the highest TOE. Use of a pen with a lower TOE requires that pen to dissipate the difference in the energy required and the energy delivered--viz., highest specified TOE--in the form of heat. The greater the variation in TOE, the greater the excessive energy, i.e., heat buildup. The amount of excess heat that a given pen can tolerate is a function of the operating temperature range and the acceptable reliability for the particular application. The relationship of TOE to the ability to dissipate heat is known as a particular pen design "energy budget." Moreover, as drop generator density increases on the printhead--e.g., from 150 nozzles to 300 nozzles in substantially the same size circuit--the ability to dissipate heat decreases. While most of the energy is carried away by the ejected ink drop, the increase in drop generator density decreases the overall energy budget.
The goal therefore is to control electrical firing pulses such that the printhead is operated at a pulse energy that is approximately at or greater than the turn-on energy of the resistor and within a range that provides the desired print quality while avoiding premature failure of the heater resistors due to variation in TOE becoming great relative to a pen's ability to dissipate heat.
There is a need to measure actual TOE for a given pen-printer combination to calculate an operating energy given an energy budget and to set dynamically a TOE-related operating energy to optimize printing operations. The variation in TOE and printers is thereby adjusted out, increasing the margin for reliability and operating temperature range, and increasing the energy budget.
In the prior art TOE determination is known to be done with thermal sensing, a process referred to as "TTOE." Referring now to FIG. 1A (PRIOR ART), shown is a simplified block diagram of a thermal ink-jet hard copy engine. A controller 11 receives print data 10 input and processes the print data to provide print control information to a printhead driver circuit 13. A controlled voltage power supply 15 provides to the printhead driver circuit 13 a controlled supply voltage, Vs, whose magnitude is controlled by the controller 11. The printhead driver circuit 13, as controlled by the controller 11, applies driving or energizing voltage pulses of voltage, VP, to a thin film integrated circuit thermal ink jet printhead 19 that includes thin film ink drop firing heater resistors 17. The voltage pulses VP are typically applied to contact pads that are connected by conductive traces to the heater resistors 17, and therefore the pulse voltage received by a resistor is typically less than the pulse voltage VP at the printhead contact pads. Since the actual voltage across a heater resistor 17 cannot be readily measured, thermal turn-on energy for a heater resistor as described herein will be with reference to the voltage applied to the contact pads of the printhead cartridge associated with the heater resistor. The resistance associated with a heater resistor 17 will be expressed in terms of pad-to-pad resistance of a heater resistor and its interconnect circuitry (i.e., the resistance between the printhead contact pads associated with a heater resistor). The relation between the pulse voltage VP and the supply voltage Vs will depend on the characteristics of the driver circuitry. For example, the printhead driver circuit 13 can be modeled as a substantially constant voltage drop, VD, and for such implementation the pulse voltage VP is substantially equal to the supply voltage Vs reduced by the voltage drop VD of the driver circuit: EQU VP=Vs-VD (Equation 1).
If the printhead driver 13 is better modeled as having a resistance, Rd, then the pulse voltage is expressed as: EQU VP=Vs(Rp/(Rd+Rp)) (Equation 2),
where Rp is the pad-to-pad resistance associated with a heater resistor 17.
More particularly, the controller 11 provides pulse width and pulse frequency parameters to the printhead driver circuitry 13 which produces drive voltage pulses of the width and frequency as selected by the controller, and with a voltage VP that depends on the supply voltage Vs provided by the voltage controlled power supply 15 as controlled by the controller 11. Essentially, the controller 11 controls the pulse width, frequency, and voltage of the voltage pulses applied by the driver circuit to the heater resistors.
The integrated circuit printhead 19 of the thermal ink jet printer of FIG. 1A (PRIOR ART) further includes a sample resistor 21 having a precisely defined resistance ratio relative to each of the heater resistors 17, which is readily achieved with conventional integrated circuit thin film techniques. By way of illustrative example, the resistance sample resistor 21 and its interconnect circuit are configured to have a pad-to-pad resistance that is the sum of: (a) 10 times the resistance of each of the heater resistors and (b) the resistance of an interconnect circuit for a heater resistor. One terminal of the sample resistor is connected to ground while its other terminal is connected to one terminal of a precision reference resistor Rp that is external to the printhead and has its other terminal connected to a voltage reference, Vc. The junction between the sample resistor 21 and the precision resistor Rp is connected to an analog-to-digital converter (A/D) 24. The digital output of the A/D converter 24 comprises quantized samples of the voltage at the junction between the sample resistor 21 and the precision resistor Rp. Since the value of the precision resistor Rp is known, the voltage at the junction between the sample resistor 21 and the precision resistor Rp is indicative of the pad-to-pad resistance of the sample resistor 21 which in turn is indicative of the resistance of the heater resistors.
The controller 11 determines a thermal turn-on pulse energy for the printhead 19 that is empirically related to a steady state drop volume turn-on energy which is the minimum steady state pulse energy at which a heater resistor 17 produces an ink drop of the proper volume, wherein pulse energy refers to the amount of energy provided by a voltage pulse; i.e., power multiplied by pulse width. In other words, increasing pulse energy beyond the drop volume turn-on energy does not substantially increase drop volume. FIG. 2 (PRIOR ART) sets forth a representative graph of normalized printhead temperature and normalized ink drop volume plotted against steady state pulse energy applied to each of the heater resistors of a thermal ink jet printhead. Discrete printhead temperatures are depicted by crosses (+) while drop volumes are depicted by hollow squares ({character pullout}). The graph of FIG. 2 (PRIOR ART) indicates three different phases of operation of the heater resistors of a printhead. The first phase is a non-nucleating phase wherein the energy is insufficient to cause nucleation. In the non-nucleating phase printhead temperature increases with increasing pulse energy while ink drop volume remains at zero. The next phase is the transition phase wherein the pulse energy is sufficient to cause ink drop forming nucleation for some but not all heater resistors, but the ink drops that are formed are not of the proper volume. In the transition phase the ink drop volume increases with increasing pulse energy, since more heater resistors are firing ink drops and the volume of the ink drops formed are approaching the appropriate drop volume, while the printhead temperature decreases with increasing pulse energy. The decrease in printhead temperature is due to transfer of heat from the printhead by the ink drops. The next phase is the mature phase wherein drop volume is relatively stable and temperature increases with increasing pulse energy. FIG. 2 (PRIOR ART) shows only the lower energy portion of the mature phase, and it should be appreciated that printhead temperature increases with increased pulse energy since ink drop volume remains relatively constant in the mature phase.
As discussed more fully in U.S. Pat. No. 5,428,376, Wade et al., assigned to the common assignee of the present invention, the sample resistor 21 can be utilized to determine the pad-to-pad resistance associated with the heater resistors in order to determine the energy provided to the heater resistors as a function of the voltage VP and pulse width of the voltage pulses provided by the driver circuit. The integrated circuit printhead of the thermal ink jet printer of FIG. 1A (PRIOR ART) also includes a temperature sensor 23 located in the proximity of some of the heater resistors, and provides an analog electrical signal representative of the temperature of the integrated circuit printhead. The analog output of the temperature sensor 23 is provided to an analog-to-digital converter 25 which provides a digital output to the controller 11. The digital output of the A/D converter 25 comprises quantized samples of the analog output of the temperature sensor 321. The output of the A/D converter is indicative of the temperature detected by the temperature sensor. The output of the temperature sensor is sampled for the different ink firing pulse energies applied to the heater resistors, for example at least one sample at each different ink firing pulse energy. For a properly operating printhead and temperature sensor, temperature data acquisition by stepwise pulse energy decrementing and temperature sampling continues until it is determined that acceptable temperature data has been produced. TTOE for a target drop volume is calculated accordingly.
Another prior art method of measuring TOE for the ejection of ink drops is known as visual turn-on energy, "VTOE," process. A pattern comprising lines printed by each of the pen's nozzles of one or all colors is printed at a known energy setting. The energy is decremented a known amount and a nozzle pattern is printed adjacent to the previous pattern. Continuing in this fashion, eventually the energy level is reached in which a substantial number of the nozzles (usually greater than ten percent) are no longer being printing. The TOE level corresponding to the last area that did print as a complete patten is selected by the observer, either during final manufacturing test phase or by the end-user.
Yet another prior art method is the use of electrostatic discharge as a method of TOE measurement. A charged plate is mounted in a printer service station such that as ink drops hit the plate a charge transfer can occur, generating a current. By firing ink drops at increasing energy levels, the onset of a current flow determines the TOE.
There is a need for a method for determining turn-on energy that is independent of both printhead thermal response and subjective observer analysis and intervention. There is a need for a method and apparatus that calibrates turn-on energy relative to actual print data. Moreover, there is a need for an automatic calibration of printhead turn-on energy and an appropriately related printhead operation energy that can be instigated without end user intervention.